The use of braking valves is well known for controlling hydraulic components bearing heavy loads. These braking valves are designed so as to avoid a sudden and dangerous drop in the load lifted in the event of a break in a hydraulic oil carrying pipe.
Thus in the case of a double-acting hydraulic jack, a braking valve is connected in series a pipe which communicates with the main chamber of the jack when this main chamber is placed under pressure to raise the load. This braking valve is then governed by the pressure in the annular chamber of the jack, and the braking valve can only open in proportion with the degree to which the annular chamber is put under pressure. Each braking valve is generally mounted in parallel with a non-return valve which allows hydraulic fluid to pass only in the direction of the main chamber of the corresponding jack, assuming that the braking valve offers only a reduced section for the passage of the fluid even when fully open.
The known prior art braking valves may be used only with hydraulic circuits in "series distribution", that is to say, with hydraulic circuits including hydraulic distributors grouped together in a single assembly, the return circuit of each distributor being connected to the input circuit of the following distributor, and so on. The hydraulic circuits in "series distribution" present two major advantages.
First, certain interdependence of the different movements of a piece of equipment is obtained. This is advantageous, such as, in the case of a machine such as an excavator with a pivoted arm. As for example, it is advantageous when it is desired to work on any surface with a component placed at the free end of an arm with the pivoting of each arm component being controlled by a particular hydraulic jack.
Second, the use of a given valve as an output or pressure amplifier with regard to the following jack is allowed by varying the relative sections of the hydraulic jacks.
On the other hand the "series distribution" entails variations in pressure in the return circuits during the simultaneous operation of several distributors. There is the disadvantage that these pressure variations disturb the operation of the breaking valves.
The present invention has the aim of achieving a braking valve which may be used in hydraulic circuits in "series distribution" without experiencing the disadvantage of pressure variations.
The present invention also has the aim of achieving a braking valve which may be fitted directly on to the hydraulic jacks. The jacks are mounted in such a way that the connection of the hydraulic circuit between the braking valve and its hydraulic jack is direct and does not pass through any external piping which is prone to breaking or failure.
Finally, the present invention has the aim of achieving a breaking valve which allows a controlled descent of the load in the case of a break in the piping connected to the main chamber of a hydraulic jack.
A breaking valve according to the invention includes a seat situated in the body of the valve between a first chamber, or front chamber, connected to the inlet opening of a hydraulic component to be controlled, and a second chamber, or intermediate chamber, connected to an outlet of the distributor associated with the component.
A blocking component which is able to block the seat by moving forward, that is to say, in the direction of the first chamber.
A governing piston is disposed co-axial with the seat, and includes on the one hand a front cylindrical bearing surface mounted to slide in a first bore of the valve body, and on the other hand a rear cylindrical bearing surface mounted to slide in a second bore in the valve body, of greater diameter than the first bore, the first bore opening to the front into the second chamber, while the front end of the governing piston bears against the rear of the blocking component.
A third chamber, or rear chamber, of annular shape, is formed in the body of the valve in front of a shoulder of the governing piston located between the two cylindrical bearing surfaces thereof. This third chamber is connected to a governing circuit.
A calibrating spring is compressed against the rear of the governing piston so as to tend to permanently press the blocking component on the seat. Furthermore the contact between the blocking component and the seat takes place along a line which has the same diameter as the first bore, the longitudinal balance of the blocking component and the governing piston are thus independent of the pressure obtained in the second chamber, and depend only on the recall force of the calibrating spring, the force of pressure in the first chamber, and the force of pressure in the third chamber.
According to an additional characteristic of the invention, the valve body is made so as to bolt directly on to the body of a hydraulic jack, the first chamber communicating directly with the inlet opening of one of the chambers of the jack body.
According to an additional characteristic of the invention, the blocking component includes, at its front end, a cylindrical bearing surface mounted to slide, with little play, in a blind bore which opens into the first chamber, the cylindrical bearing surface forming a dampening chamber in the bottom of the blind bore to dampen the movements of the blocking component.
According to an additional characteristic of the invention, the valve in addition includes a non-return device integral with the body of the valve, and provided with a movable blocking component which allows the passage of fluid only from the intermediate chamber to the front chamber.